The tools of his trade were simple enough; he made calls to banks from an internet cafe, putting on different accents to bamboozle staff, who believed he was an executive seeking an extension for an overdraft.

But for Frageand Nassem, that was all he needed to commit remarkable fraud.

Over four years, the 32-year-old managed to swindle £8.2m out of high street banks, adopting Scottish, Pakistani and other voices as he impersonated executives and bank managers.

Using information he gleaned from Companies House, he tricked banking giants, including HSBC and RBS, to transfer funds from accounts with existing clients into accounts he'd set up for himself using bogus details.

And then the money disappeared. Where it went, or what it was spent on, remains a mystery to this day.

Nassem, from Edinburgh, took such delight in his tricks that he'd call his victims afterwards and laugh at them – having made sure that he'd left no tracks for the banks to follow. He was eventually caught when he attempted an even more outrageous fraud – he tricked foreign currency traders at RBS to use an account run by the Bank of Korea to buy $170m. But he was unable to unlock the cash for himself after he failed to find a way of opening a dollar account to access the funds.

For a man with such an ingenious criminal mind, he was caught out by the simplest of mistakes – he rang his mum from a phone he used to dupe the banks.

Yesterday Nassem was jailed for nine years at Croydon crown court and was warned by Judge Stowe that he faced a further 10 years inside unless he revealed exactly what he has done with the money he stole.

Police described him as a "smart, articulate and arrogant young man" who understood the inner working of international banks and how they could be defrauded of funds. The long term and often subtle methods he used to extract small amounts of information were "extraordinary".

Detectives strongly suspect he was responsible for a series of other similar frauds.

But though police said yesterday they were delighted to have caught Nassem, they still have no idea where the cash has gone.

He spent little of the money on himself. He maintained a five-bedroom house in Livingston, Scotland and leased a Range Rover, but there was little evidence of a lavish lifestyle. Millions of pounds remain unaccounted for, the police said, mostly, it is believed, in foreign bank accounts.

Speaking after the verdict, Superintendent Colin Cowan of the City of London Police said: "I am very pleased to see the level of sentence given to Nassem. It shows once again that fraud is a serious crime with serious consequences."

Nassem was arrested in Edinburgh in 2006 by City of London Police and charged with four major frauds.

He was traced to an internet cafe, which had been identified as the source of a number of emails relating to the frauds.

When searched he was found in possession of a mobile phone that had been used to contact another bank he was targeting, and a notebook containing contact details for staff at that company.

After his arrest he told officers he was mentally ill. While on remand he was seen by several psychiatrists and for more than two years considered too ill to stand trial.

His defence team argued he suffered from paranoid schizophrenia.

But a decision by the crown prosecution service to press ahead with the trial was backed by Judge Stowe, who said he was faking his illness and had held up the trial by his "stalling".

City of London Police, which take the lead in investigations of financial crime, especially when it affects banks, said: "Nassem was so convincing with his use of a range of accents and his persistent nature that he hoodwinked staff in several banks into believing he was calling from the client company.

"He was also known to call back and laugh at them down the phone once he had stolen the funds.

"His downfall came about because he made two key mistakes: he couldn't resist calling his mum from his criminal phone and he used his library card to access an internet cafe."